ACI STRUCTURAL JOURNAL TECHNICAL PAPER

Title no. 101-571

Near-Surface-Mounted Fiber-Reinforced Polymer

The use offiber~rejllforced polymer (FRP) materials for strength- prorrusmg technique for strengthening masonry walls andening bridges and buildings has been used extensively in the last reinforced concrete members. Design guidelines for thisdecade. FRP has been used in different configurations and techniques technique are currently under consideration by ACIto use the moten"at effectively and to ensure long service life of the Committee 440 for the coming version of the "Guide for theselected system. One of these innovative strengthening techniques isthe near-surface mounted (NSM) that consists of placing FRP Design and Construction of Externally Bonded FRP Systemsreinforcing bars or strips into grooves precut into the concrete cover for Strengthening Concrete Structures (ACI 440.2R-02)." Thein the tension region of the strengthened concrete member. This NSM reinforcement technique consists of placing the FRPmethod is relatively simple Gild considerably enhances the bond 0/ reinforcing bars or strips into grooves precut into the concretethe mounted FRP reinforcements, thereby using the material more cover in the tension region of the reinforced concrete membereffectively. This paper presents test results of reinforced concrete and bonded to the three sides of the groove using high-strengthT-beams strengthened in flexure with different strengthening systems epoxy adhesive or cementitious grout.using FRP reinforcing bars and strips as NSM reinforcement and The application of NSM FRP reinforcement does notexternally bonded FRP strips, The FRP reinforcements used in thisinvestigation include carbon fiber-reinforced polymer (CFRP) requires surface preparation work as in the case of externallyreinforcing bars alld strips and glass fiber-reinforced polymer bonded FRP reinforcement. In addition, the NSM FRP(GFRP) thennoplastic strips, The behavior and effectiveness of the strengthening technique is also very efficient and practicalmaterials used for the various strengthening systems are compared. for flexural strengthening of slabs and beams in the negativeThe structural performance and modes offailure of the tested beams moment regions. Use of externally bonded FRP reinforcement inare presented and discussed Test results indicated that using NSM such cases could be subjected to mechanical and envirorunentalFRP reinforcing bars and strips is practical, significantly improves damage and would require extensive protective cover that couldthe stiffness, alld increases the flexural capacity of reinforced interfere with the presence of floor finishes. Configuration of theconcrete beams, The limitations of using NSM FRP reinforcing bars FRP reinforcements used for the NSM technique is controlled byand strips are controlled by serviceability requirements ill tenns of the depth of the concrete cover. After installation, the NSM FRPoverall deflections and crack widthr rather than delamination,observed by many researchers, of externally bonded FRP reinforcements are protected against mechanical damage, wear,reinforcement. Strengthening of reinforced concrete beams using NSM impact, and vandalism from vehicles. The technique could alsoFRP strips provided higher strength capacity than externally bonded provide better fire resistance in the event of a fire; therefore, itFRP strips using the same maten'al with the same axial stiffness. could reduce the cost of fire protection measures.

ACI Structural Journal/September-October 2004 717

The performance of various NSM FRP reinforcing barsACT member Raafat EI-Hacha is an assistant professor in the Department of CivilEllgineering, University of Calgary, Calgary. Alberta, Canada. He received his PhD and strips, as well as externally bonded FRP sheets on small-in civil engineering from Queen's University, Kingston, On/aria, Canada, ill 200}. He scale concrete beams and slabs, was investigated by Hassanis a member of ACI Committee 440, Fiber Reinforced Polymer Reinforcement, alld (2002), including cost analysis for each of the FRP-Joint ACI-ASCE Committee 423, Prestressed Concrete. His research interests includethe application of memally prestressed wuJ nonprestressed fiber-reinforced polymer strengthening techniques. Test results showed that using(FRP) sheets and strips, memnlly prestressed FRP cables, and near-suiface-mounted NSM CFRP reinforcing bars increased the strength by 36%.FRP strips and reinforcing barsfor strengthening concrete structures. Using NSM CFRP strips increased the strength by 43% inSami H. Rizkalla, FACI, is a Distinguished Professor of Civil Engineering and comparison with an increase of only 11 % using the axialConstruction, Director of the COllstructed Facilities lAboratory (CFL) at North stiffness used as externally bonded strips due to peelingCarolina State Universit)" Raleigh, N.c., and Director of the NSF IliduslrylUniversityCooperative Research Center. He is a past Chair of ACI Committee 440, Fiber- failure of the strips. Hassan (2002) reported that the efficiencyReinforced Polymer Reinforcement. and a member of Joint ACl·ASCE Committee of using FRP reinforcing bars as NSM reinforcement is550, Precast Concrete Structures. controlled by the bond characteristics of the reinforcing bars in addition to the bond between the epoxy adhesive materialcompared with externally bonded FRP strips using the and the surrounding concrete in the groove. Such behaviorsame material and axial stiffness . The findings of this has been confirmed and reported recently by otherresearch provide data for the design guidelines currently researchers (De Lorenzis and Nanni 2002) . The maximumunder consideration by ACI Committee 440 for the NSM tensile strain in the CFRP and GFRP bars used as NSM rein-FRP strengthening technique. forcement did not exceed 33 and 60% of the rupture strain of the bars at failure, respectively (De Lorenzis and Nanni 2002). Hassan (2002) reported that such a limiting value is BACKGROUND highly dependent on the configuration and the ratio of the Published literature on the use of NSM FRP for structural steel reinforcement inside the concrete beam as well as onstrengthening is very limited when compared with that of the stress level at the concrete-epoxy interface. The authorexternally bonded FRP laminates. Even though the use of found that the maximum measured tensile strain in the CFRPNSM FRP reinforcement for strengthening is relatively bars at failure is in the range of 40 to 45% of the rupturerecent, NSM steel bars were used in Europe for strength- strain of reinforcing bars, and that the rupture of CFRPening of reinforced concrete structures. Asplund (1949) reinforcing bars is not likely to occur regardless of thecarried out tests on concrete beams strengthened with NSM embedment or bond length or the type of epoxy adhesive used.steel bars grouted into diamond-sawed grooves filled with Recently, prestressed NSM CFRP rectangular rods havecement mortar and compared their behavior with that of been used as a bonded post-tensioned system to strengthenconventional concrete beams reinforced with steel bars. concrete beams (Nordin, Tiiljsten, and Carolin 2001). TheIdentical behavior for both sets of specimens was observed. initial strain was approximately 0.002, which is about 12% ofThe same technique was used in strengthening a reinforced the ultimate strain. The beam strengthened with prestressedconcrete bridge deck in Sweden that experienced excessive NSM CFRP rods showed about a 100 and 37% increase in thesettlement of the negative moment reinforcement during cracking and yielding load, respectively, compared with theconstruction, so that the negative moment capacity needed to beam strengthened with nonprestressed NSM CFRP rods. Thebe increased (Asplund 1949). The advantage of using FRP ultimate loads and failure modes were the same with orinstead of steel is primarily due to its corrosion resistance, without prestress by rupture of the NSM FRP rods; however,which is particularly important in this case due to the location beams prestressed with NSM CFRP rods had smallerof the reinforcing bars or strips being very close to the deflections at failure.surface that could be exposed to aggressive environmentalattacks (De Lorenzis and Nanni 2002). Alkhrdaji et al. EXPERIMENTAL PROGRAM(1999) conducted in-place tests on reinforced concrete Test specimens and setupbridge decks strengthened with NSM sandblasted CFRP A total of eight, simply supported, 2.7 m (9 ft) long,rods. Test results showed an increase in the moment capacity concrete T-beams were constructed and tested under aof 27% compared with the unstrengthened deck. monotonically increasing concentrated load applied at Research in Germany indicated that the bond characteristics midspan of the beam. The test setup of aT-beam specimenof NSM CFRP strips are superior to externally bonded CFRP is shown in Fig. 1. The load was applied using a closed-loopstrips (Blaschko and Zilch 1999). NSM CFRP sandblasted controller-testing machine operating using stroke-controlrods and deformed GFRP rods increased the flexural strength mode at a loading rate of 1.07 mmlmin (0.042 inlmin). Oneof simply supported reinforced concrete T-beams by 30 and beam was tested as a control specimen whereas the other26%, respectively (De Lorenzis, Nanni and La Tegola 2000). seven beams were strengthened using different FRP rein-Increasing the amount of NSM reinforcement did not produce forcements including CFRP reinforcing bars and strips assignificant gain in the capacity. Arduini, Gottardo, and well as GFRP thermoplastic strips.DeRiva (2001) found that the use of high-strength mortar with The bottom tension reinforcement consisted of two No. 13compensated shrinkage or epoxy putty guarantee full use of deformed steel bars of nominal diameter 12.7 mm (1/2 in.)the NSM FRP strengthening system. The ultimate load- running along the full length of the beams and two No. 16carrying capacity of beams strengthened with rectangular deformed steel bars of nominal diameter 15.9 mm (5/8 in.)NSM CFRP rods using epoxy adhesive and cement grout as terminated with a 90-degree bent at 100 mm (4 in.) awaybonding agent has increased by 77 and 56%, respectively from the midspan section on both sides, as shown in Fig. 1.(Tiiljsten and Carotin 2001). Using high-strength rectangular This arrangement of the bottom reinforcement was selectedNSM FRP rods and high modulus rectangular NSM FRP rods to ensure that the flexural failure of the strengthened beamincreased the ultimate load capacity by !O8 and 93%, will always occur at the midspan section and to simulaterespectively (Carolin, Hordin, and Tiiljsten 2001). field conditions where the bottom steel reinforcement is

different NSM FRP systems using CFRP reinforcing bars, Fig. i-Test setup, beam details, and instrumentation oftwo types of CFRP strips, and thennoplastic GFRP strips. beam specimens_Three beams (B2a, B2b, and B4a) were strengthened withdifferent externally bonded CFRP and GFRP strips. Beam B 1 was strengthened with one 9.5 mm (3/8 in.) Table 1-Test matrix for T-beam specimensdiameter NSM CFRP reinforcing bar (Hughes Brothers Beam no. FRP strengthening system2002). BeamB2 was strengthened with two Type 1 2x 16 mm BO No strengthening(0.078 x 0.63 in.) NSM CFRP strips (Hughes Brothers Bl One NSM CFRP reinforcing bar2002) . Beam B3 was strengthened with two Type 2 1.2 x B2 1\vo Type 1 NSM CFRP strips25 mm (0.05 x 1.0 in.) NSM CFRP strips (Structural B3 1\vo Type 2 NSM CFRP stripsComposites, Inc. 2002). Beam B4 was strengthened with five B4 Five NSM GFRP thennoplastic strips2 x 20 mm (0.078 x 0.78 in.) NSM GFRP thennoplastic strips B2a 1\vo Type 1 eXlernally bonded CFRP strips(Dow Plastics Chemical 2000). B2b Two Type 1 externally bonded CFRP strips Beams B2a and B2b were each strengthened with two B4a Five externally bonded GFRP thermoplastic stripsType 1 2 x 16 mm (0.078 x 0.63 in.) externally bonded CFRPstrips (Hughes Brothers 2002). Beam B2b was severelydamaged before strengthening. Beam B4a was strengthened ISO, and 275 mm (2.6, S.9, and 10.8 in.) from the bottomwith five 2 x 20 mm (0.078 x 0.78 in.) externally bonded surface of the concrete beam. The strains in the NSM FRPGFRP thermoplastic strips (Dow Plastics Chemical 2000). reinforcing bars and strips and the externally bonded FRP The embedment length of all the NSM FRP reinforcing bars strips at three different locations were monitored duringand strips and the length of the externally bonded FRP strips testing using three electrical resistance 120 ohms strainwere kept constant in all beams as 2400 mm (7 ft, 10-112 in.). gages. The strain gages were installed on all FRP reinforcingThe same axial stiffness (EA)FRP for all FRP reinforcement bars or strips at midspan, at 400 and 800 mm (lS.7S and 31.S in.)used in this study was kept constant, hence, according to the from the midspan on one side of the FRP reinforcing bars orclassical beam theory, the load-deflection behavior of all strips. The deflections at midspan, at the supports, and at 400strengthened beams is anticipated to be identical, where E and 800 mm from midspan on both sides of the beam wereand A are the modulus of elasticity and cross-sectional area measured using linear variable displacement transducersof the FRP reinforcement, respectively. A summary of these (LVDTs). One LVDT was placed at each location except atbeams is given in Table 1. midspan where two LVDTs were used and averaged. The slip at the free ends of the NSM FRP reinforcing bars and stripsInstrumentation was measured using two LVDTs. The data were automati- The beams were instrumented, as shown in Fig. I, to cally collected. The location of cracks and their propagationmeasure applied load, deflection, and strain in the concrete was clearly marked on both sides of the beams. Crack widthsand in the FRP reinforcement during testing. The concrete were also measured at every 4.5 kN (1.0 kips).strains in the compression zone at the top surface of thebeams were measured using two displacement transducers Fiber-reinforced polymer strengthening systems(DT) placed at 100 mm (4 in.) from the midspan on both Four products, provided by three manufacturers (Mant.-I tosides of the beam. In addition, three DT gages were installed Manf.-3) were investigated. Product-l = Asian 200 CFRPin the midspan zone along the front face on one side of the reinforcing bars (Manf.- l = Hughes Brothers 2002);beam centerline to measure the strain distribution in concrete Product-2 = Asian 500 CFRP strips (Manf.-l = Hughesover the depth of the beam. The DT gages were placed at 65, Brothers 2002); Product-3 = unidirectional pultruded laminate

., . completely with the appropriate epoxy adhesive paste

It 1 iiii using a manual epoxy gun to provide the necessary bond with the surrounding concrete. Then the FRP reinforcements I -A-II-dim-~-'-if)I1S-0-~-'"-m-m--- 2400 - - - - -- ---->; (reinforcing bars and strips) were inserted inside the grooves ensuring that they were completely covered with epoxy and lightly pressed to displace the bonding agent. This action

".Y 6ATF .AM .AIr

- 1,,1+- I NSM CFRP rchar <19.li"J 2 NSM CFRP strips type I - 1,,1+- 2 NSM CFRP strips type 2 _ 11+-38 5 NSM GFRP strips forces the epoxy paste to flow around the FRP reinforcement and completely fill the space between the FRP and the sides of the groove, The groove was then filled with more paste if needed and the surface was leveled, The excess adhesive was removed with a spatula. The surface was smooth finished to achieve uniform distribution. The same procedures in termsFig. 2- Groove dimensions for various NSM FRP of cutting the groove, injecting the epoxy, and placing thestrengthening systems. FRP reinforcing bars and strips were applied. The epoxy adhesive was allowed to fully cure at room temperature for atCFK 150/2000 CFRP strips (Manf.-2 = Strucrural Composites, least 1week before testing of the bearns.Inc. 2002); and Product-4 = thermoplastic GFRP strips(Manf,-3 = Dow Plastic Chemical 2000). The GFRP strips Dimensions of groovesare manufactured using the thermoplastic composite The grooves cut at the bottom surface of the concretetechnology, which enables the production of high-performance beams had different cross sections depending on the type ofunidirectional pultruded composites based on a thennoplastic FRP reinforcements used as shown in Fig, 2. For the 9.5 mmmatrix with continuous fiber reinforcement. The material (3/8 in.) NSM CFRP reinforcing bar, the groove wasproperties of the different FRP strengthening systems are approximately 18 mm (0.708 in.) wide and 30 mm (1.18 in,)given in Table 2 as reported by the manufacturers with linear deep cut at the middle of the bottom width of the beamstress-strain behavior up to failure. specimen. For the Type 1 2 x 16 mm (0.078 x 0.63 in.) NSM Commercially available epoxy adhesives were used for CFRP strip, two grooves, 75 mm (3,0 in.) apart, were cut atbonding the FRP reinforcing bars and strips. A two-component the bottom width of the beam specimen; each groove wasadhesive with a mixture ratio of 1 (resin) : 1 (hardener) by approximately 6.4 mm (0.25 in.) wide and 19 mm (3/4 in.)volume was used for bonding the NSM CFRP reinforcing deep, For the Type 21.2 x 25 mm (0.05 x 1.0 in.) NSM CFRPbars to the surrounding concrete (ChemCo Systems, Inc, strip, two grooves, 75 mm (3,0 in.) apart, were cut at the1995). As reported by the manufacturer, the adhesive has a bottom width of the beam specimen; each groove wasmodulus of elasticity of 1200 MPa (174 ksi), an ultimate approximately 6.4 mm (0.25 in.) wide and 25 mm (1.0 in.)tensile strength of 48 MPa (6,96 ksi), a compressive yield deep. For the 2 x 20 mm (0.078 x 0.78 in.) NSM GFRPstrength of71.7 MPa (10.4 ksi), and a compressive modulus strip, three grooves, 38 mm (1.5 in.) apart, were cut at theof 3378 MPa (490 ksi). Another type of high-strength epoxy bottom width of the beam specimen; each groove wasadhesive was used to bond the CFRP and GFRP strips to the approximately 6.4 mm (0.25 in.) wide and 25 mm (1.0 in.)concrete and consisted of a two-component adhesive with a deep, One GFRP strip was inserted into the middlemixture ratio of 2: 1 by volume (Structural Components, Inc. groove and two strips bonded together side by side were2002). As reported by the manufacturer, the adhesive has a placed in each of the outer two grooves.modulus of elasticity of 3500 MPa (507.6 ksi), an ultimatetensile strength of 70 MPa (10.2 ksi), and a compressive Installation of externally bondedstrength of 82.7 MPa (12 ksi), FRP reinforcements To ensure a good, strong bond, the bottom surface of theInstallation of NSM FRP reinforcements concrete beams was prepared by grinding until the coarse Installation of the NSM FRP reinforcing bars and strips aggregates were exposed, then cleaned by washing and air-begins by making a series of grooves with specified dimensions brushing to remove dust or debris and fine particles. Carecut into the concrete cover in the longitudinal direction at the was taken to ensure that the resulting concrete surface aftertension side of the beam specimens. A special concrete saw grinding was unifonn. Following cleaning, a unifonn 2 mmwith a diamond blade was used to cut the grooves with the (0.078 in.) thin layer of the two-part epoxy-based adhesivedimensions shown in Fig. 2. The grooves were cleaned from was applied by palette knife to the bottom surface of theany dust and air-brushing pressure was used to remove concrete beam. The FRP strips were placed in position on thedebris and fine particles to ensure proper bonding between concrete surface and pressed onto the epoxy by hand, Tothe epoxy adhesive and the concrete. ensure a good bond with concrete, a uniform pressure was The adhesive was applied into the groove before inserting applied along the entire length of the strips, AU-shapedthe FRP reinforcing bars or strips, Each groove was filled wrap CFRP sheet with 100 mm (4,0 in.) width was placed

Beam BO: Unstrengthened

Beam B1: 1 NSM CFRP Rebar - - ~--

o~--~~~~----~--~--~~--~~ 0 o 10 20 30 40 50 60around the web of the concrete beams at both ends of the Midspan Deflection (mm)

externally bonded FRP flexural reinforcements, with the

direction of the fibers perpendicular to the longitudinal axis Fig. 4- Load-midspan deflection of beams strengthenedof the member, to improve the anchorage of the FRP with NSM CFRP reinforcing bar and strips.strengthening system (Fig. 3). The externally bonded CFRPstrips (Type I) were placed at the bottom surface of theconcrete beams (B2a and B2b) with spacing in between FRP reinforcements. The un strengthened control beamequal to twice the width of the strip (Fig. 3). The externally failed by crushing of the concrete after yielding of the steelbonded GFRP strips were placed side by side at the bottom tension reinforcement.surface of the concrete beam (B4a) leaving 25 mm (LO in.)from each side of the beam (Fig. 3). Effectiveness of NSM CFRP reinforcing bars and strips TEST RESULTS AND DISCUSSION The load-midspan deflection behavior of the strengthened A summary of significant test results describing the flexural beams with NSM CFRP reinforcing bars (Beam BI) andbehavior of all tested beams is presented in Table 3. The strips (Beams B2 and B3) in comparison with theconcrete compressive strength when the beams were tested unstrengthened control beam (BO) is shown in Fig. 4. Priorwas detennined according to ASTM C 39-01, using three to cracking, the load-deflection behavior for all strengthenedstandard concrete cylinders and ranged between 48 MPa beams was similar to that of the unstrengthened beam. This(6962 psi) for Beams BO, BI, B3, and B4. and 57 MPa behavior indicates that using NSM FRP reinforcements did(8267 psi) for Beams B2, B2a, B2b, and B4a. Beam BO was not contribute to increasing the stiffness and strength in thetested without strengthening and used as a control specimen elastic range. After cracking, however, the flexural stiffnessfor comparison purposes to evaluate the improvement in flexural and strength of the strengthened beams with NSM FRP rein-strength provided by the various NSM and externally bonded forcements were significantly improved compared with the

ACI Structural Journal/September-October 2004 721

of the CFRP reinforcing bars at failure and the smaller bonding surface of the NSM CFRP reinforcing bars with respect to the NSM CFRP strips (Types I and 2). Using NSM FRP reinforcement resulted in a significant reduction of the deflection and crack widths and delayed formation of new cracks in the strengthened beams. The formation of cracks followed a typical crack pattern of flex- ural members. The first flexural crack occurred at the midspan of the beam directly under the location of the applied load. Under further increase in the load, the cracks became wider and new flexural cracks started to initiate. Many uniformly distributed cracks of small widths were observed along the full length of the strengthened beams that were symmetric about their midspan. whereas fewer cracks of greater width were observed in the unstrengthened control beam. This behavior indicates that complete bond of the FRP materials, even after yielding of the steel reinforcement, controlled crack widths and their distribution along the span. ,, As the load approached failure stage, the flexural stiffness decreased further until the ultimate load was reached and I failure occurred. Failure of Beam B I was due to splitting of the epoxyFig. 5-Debonding failure of Beam B1 strengthened with cover in the groove followed by complete debonding of theNSM CFRP reinforcing bar. reinforcing bar at the CFRP-epoxy interface and cracking of the concrete surrounding the epoxy in the groove as shown in Fig. 5. This type of failure is categorized as epoxy split failure. Initiation of the crack in the epoxy was accompanied by a distinct noise followed by progressive cracking of the epoxy paste. Longitudinal splitting cracks, which developed in the epoxy cover, led to the loss of bond of the NSM CFRP reinforcing bars. After debonding, the load dropped to a load level equivalent to the measured yielding load and the deflection kept on increasing until failure occurred due to crushing of the concrete in the compression zone-then the test was stopped as shown in Fig. 4. The debonding initiated at the concrete section where 39% of the bottom flexural reinforcement was terminated (Fig. I). Splitting of the epoxy is the result of high tensile stresses at the CFRP reinforcing bar-epoxy interface. It has been reported that to reduce the induced tensile stresses at the FRP-epoxy interface, the thickness of the epoxy cover must be increased andior high tensile adhesive must be used to shift the location of failure to the concrete-epoxy interface (Hassan 2002). Increasing the thickness of the adhesive will reduce the shear deformation within the adhesive layer and therefore results in a significant increase in debonding loads. De Lorenzis and Nanni (2002)Fig. 6-Rupture failure of Beam B2 strengthened with two reported that increasing the groove size and the cover thicknessNSM CFRP strips Type 1. leads to higher bond strength when failure is controlled by splitting of the epoxy cover.un strengthened beam. After cracking, a nonlinear behavior The failure of Beams B2 and B3 was due to rupture of thewas observed up to failure. NSM CFRP strips at midspan as shown in Fig. 6 and 7, In general, the behavior of the strengthened beams indicated respectively. After rupture of the NSM CFRP strips, the loada significant increase in the stiffness and strength in comparison dropped to a load level equivalent to the yielding load of thewith the unstrengthened beam. Using the same axial stiffness cross section and the beam behaved as conventional concreteof CFRP reinforcement, an increase in the ultimate strength beams reinforced with steel bars as shown in Fig. 4.of 69, 79, and 99% were measured for Beams B I, B2, and The load-tensile strain behavior of the NSM CFRPB3, respectively. The significant increase in ultimate load- reinforcing bars and strips is linear up to cracking of thecarrying capacity of Beam B3 strengthened with NSM CFRP concrete as shown in Fig. 8. At the onset of cracking, astrips (Type 2) compared with Beam B2 strengthened with significant increase in the measured tensile strain wasNSM CFRP strips (Type I) is due to the high ultimate tensile observed for all tested beams measured by the strain gagestrength of the material used in this case as well as to the attached to the NSM CFRP reinforcing bars or strips. Atthinner Type 2 versus Type I strips, which reduce the risk of failure, the maximum measured tensile strain in the NSMdelamination. Beam B I with NSM CFRP reinforcing bars CFRP reinforcing bars prior to debonding was 0.88%, whichshowed a smaller increase in strength due to early debonding is 77% of the rupture strain of the CFRP reinforcing bar. The

Fig. 8-Load-tensile strain in NSM CFRP reinforcing bar

and strips.maximum measured tensile strain in Types 1 and 2 CFRPstrips at failure for Beams B2 and B3 were 1.34 and 1.38%,respectively, as shown in Fig. 8, indicating full use of thetensile strength of the two types of CFRP strips used. Fig. 10-Debonding failure of Beam B2a strengthened withNSM versus externally bonded CFRP strips two externally bonded CFRP strips Type 1. Beam B2a, strengthened with externally bonded CFRP unstrengthened beam due to debonding failure of the externallystrips (Type 1) exhibited similar behavior to that of the bonded strips from the concrete surface. However, as shownunstrengthened control beam up to cracking. This behavior in Fig. 9, the NSM CFRP strips (Type I) increased theindicates that using externally bonded FRP reinforcements strength by 79%. Therefore, the strength increase using thedid not contribute significantly to the stiffness and strength same CFRP strips as NSM was approximately 4.8 times thatin the elastic range. After cracking, the load-deflection obtained using externally bonded strips. Another beam, B3a,response of the beam strengthened with externally bonded strengthened with two externally bonded CFRP strips (Type 2),CFRP strips followed the same behavior of beams strengthened was tested and achieved a 25% increase in strength and failed bywith NSM CFRP strips up to yielding of the flexural reinforce- debonding of the CFRP strips from the concrete at maximumment as shown in Fig. 9. After yielding of the internal steel rein- measured tensile strain in the strips of 0.42% (EI-Racha et al.forcement, and under further increase of the applied load, the 2004). Thus, the NSM strengthening technique using CFRPcracks continued to widen and failure occurred due to strips is more effective than the externally bonded one.debonding of the externally bonded strips as shown in Fig. 10. The load-tensile strain behavior of the CFRP strips was Using the same axial stiffness (EA)FRP of the CFRP strips similar for Beams B2 and B2a until debonding occurred forused as NSM for Beam B2, externally bonded CFRP strips the externally bonded CFRP strips as shown in Fig. 11. Atincreased the strength by only 16.6% compared with the the onset of delamination, the maximum measured tensile

20 debonding of the CFRP strips from the concrete substrate.

Z 80 16 .- ~ The maximum measured tensile strain in the CFRP strips was 0.44%, as shown in Fig. 11, indicating that only 39% of"- ~~ •0 60 ii0 the rupture strain reported for the CFRP strips was used. This ....•~ 12~ ~ ~ measured strain is 83% of the strain limitations recommended.!1: 40 by ACI 440.2R (2002) to prevent debonding of the CFRP

~i« 8 ~

Beam BO: Unsuengthened

« strips at the ultimate-limit state. 20 ..... Beam 82: 2 NSM CFRP Strlps-' 4 Beam 83: 2 NSM CFRP Strlps 'Y1>oZ Beam 84: 5 NSM GfRP Strips 82.:83, and 54 ! Effect of material type of fiber 0 0 Beam B4, strengthened with GFRP thermoplastic strips as 0 10 20 J<) 40 Midspan Oeflection emm) 50 60 NSM reinforcement, exhibited significant enhancement in strength and stiffness in comparison with the unstrengthenedFig. 12-Load-midspan deflection of beams strengthened beam as shown in Fig. 12. An increase in the ultimatewith NSM CFRP and thermoplastic GFRP strips. strength of 85% was observed. The failure of Beam B4, due to cracking of the concrete surrounding the epoxy in thestrain in the externally bonded CFRP strips was 0.48% for groove, occurred at the concrete-epoxy interface known asBeams B2a as shown in Fig. II. This represents only 44% of "concrete split failure" as shown in Fig. 13. Debondingthe rupture strain reported by the manufacturer for the CFRP started to occur at the location where 39% of the bottom steelstrips; therefore, the externally bonded strengthening system reinforcement was terminated as shown in Fig. 1. Thisdid not use the full tensile strength of the CFRP strips. The failure is the result of the high shear stress concentration inmaximum measured tensile strain in the externally bonded this zone as discussed previously. Debonding of the concreteCFRP strips at failure, Beam B2a, is 36% of the strain and the split failure occurred when the tensile stresses at themeasured by using the same type of strips as NSM reinforce- concrete-epoxy adhesive interface reached the tensilement for Beam B2 as shown in Fig. II. It should be noted strength of concrete. This failure mode is greatly influencedthat the measured strain at debonding of the strips is 91 % of by the groove dimensions as well as the mechanical charac-the strain limitation recommended by ACI 440.2R (2002) to teristics of the materials (De Lorenzis and Nanni 2002;prevent debonding of the FRP at the ultimate-limit state. Hassan 2002). Debonding extended as a horizontal splitting crack along the concrete cover toward the ends of the beam.Strengthening of severely damaged beam Under further increase of the applied load, the horizontal A severely damaged reinforced concrete beam due to split crack became wider and extended into the end of theimpact load beyond cracking strength, Beam B2b, was NSM GFRP strips causing severe cracking in the concretestrengthened using externally bonded Type 1 CFRP strips. In cover. At the onset of failure, the load dropped to the yieldingcomparison with control Beam BO, the load-deflection load level of the beam cross-section and the deflection kept onresponse of retrofitted Beam B2b was improved as indicated increasing until failure occurred due to crushing of theby the increase in stiffness and strength of the beam as shown concrete in the compression zone. After complete failure, thein Fig. 9. The behavior of retrofitted Beam B2b was similar concrete cover to the intemal steel separated from the steel.to that of undamaged, strengthened Beam B2a. The externally It was observed that the NSM GFRP strips had adhered wellbonded CFRP strips were capable of restoring the original to the concrete of the strengthened beam. The amount ofstiffness and increasing the load-carrying capacity of the concrete adhered to the GFRP strips varied considerably. Inseverely damaged beam and at the same time to approach the examining the concrete surface of the failed member,ultimate capacity of undamaged, strengthened Beam B2a. aggregate pullout was noted without any sign of damage ofThe failure of Beam B2b was similar to Beam B2a due to the NSM GFRP strips.

It should be noted that using the same axial stiffness for

strengthening Beam B4 with five NSM GFRP thermoplasticstrips exhibited sintilar load-deflection behavior as Beam B2strengthened with two Type I NSM CFRP strips up to failureof Beam B2 as shown in Fig. 12. The load-tensile strain behavior of the NSM CFRP strips(Types I and 2) was similar to the NSM GFRP thermoplasticstrips as shown in Fig. 14. As presented previously, failureof the NSM CFRP strips (Types I and 2) occurred by ruptureof the strips when the strain in the strips reached the ultimatetensile strain capacity reported by the manufacturers (1.12%for Type 1 CFRP and 1.34% for Type 2 CFRP). These ultimatestrain capacities are significantly less than the 2.2% ultimatetensile strain capacity of the GFRP thermoplastic strips asreported by the manufacturer. Because the GFRP thermo-plastic strips possess large ultimate strain capacity and Fig. i6-Debonding failure of Beam B4a strengthened withbecause the thickness of the epoxy in the outer grooves was five externally bonded thermoplastic GFRP strips.almost half that of the epoxy in the ntiddle groove, failure wasdontinated by the higher shear stresses at the concrete-epoxyinterface. Therefore, increasing the thickness of the epoxy 100

(that is, increasing the groove size) will reduce the shear 20stresses at the concrete-epoxy interface and could result in an 60 .-• '''''' (

increase in debonding load. This has been confirmed by 16

De Lorenzis and Nanni (2002) and Hassan (2002). a ~

Beam 84: 5 NSM GFRP StrIps

• 12 ..3 Beam 84a: 5 EB GFRP Strips ~

NSM versus externally bonded GFRP strips •

The ultimate load-carrying capacity of Beam B4a . . •" • ~

strengthened using externally bonded thermoplastic GFRP

strips increased by 28%. Beam B4a showed similar 20 '~ ~ "" ' . .- .-.a _.-=-=-- --: -.,~.--'. . . ~ 4behavior compared with Beam B4 up to a load level of 66 : S4 . i a4. 'kN at which debonding of the externally bonded GFRP o~~~~~~~~~~~~--~--~ astrips occurred as shown in Fig. 15. Debonding of the M ~ U M M ,. 12 1.4 Tensile Strain In GFRP Strips at MIdspan ("I.)externally bonded GFRP strips involved separation of thestrips from the concrete substrate in the form of adhesive Fig. i7- Load-tensile strain in NSM and extemally bondedshear failure at the concrete-adhesive interface (interfacial thennoplastic GFRP strips.failure) as shown in Fig. 16. At failure, the externally bondedGFRP strips slipped instantaneously from the end anchorages. tensile strain of the GFRP strips was 50% less than theA more ductile behavior was observed in Beam B4 maximum limit specified by ACr 440.2R (2002) to preventcompared with Beam B4a. debonding of the FRP at ultimate. The results indicate that The maximum measured tensile strain at failure of the the ACr 440.2R equation for strain lintitation to avoidNSM GFRP strips and externally bonded GFRP strips were debonding should be reexantined.approximately 1.35 and 0.62%, respectively, as shown inFig. 17. The measured strain is 61 and 28% of the rupture CONCLUSIONSstrain of the GFRP strips. At the onset of delamination of the The effectiveness of using near-surface-mounted FRPexternally bonded GFRP strips, the maximum measured reinforcing bars or strips for strengthening concrete beams

ACI Structural Journal/September-October 2004 725

I has been illustrated. The following observations and ACKNOWLEDGMENTS conclusions can be drawn from the experimental results: The authors would like to thank the technical staff at the Constructed Facilities Laboratory at North Carolina State University and J. N. da Silva 1. Strengthening with NSM FRP reinforcing bars or strips Filho for their help with the laboratory work. The authors are grateful to the improved the load deflection response of the reinforced support provided by Hughes Brothers and Dow Chemica] Co. for donating the concrete beams. The use of NSM FRP reinforcements FR.P materials. The authors would like to thank. T. Hassan for designing and enhanced the flexural stiffness and significantly increased constructing the beams during his PhD studies at the University of Manitoba. The authors wish to acknowledge the support of the Networks of Centres of the ultimate load-carrying capacity of the strengthened Excellence Program of the Government of Canada and the Natural concrete beams. The difference in the behavior prior to Sciences and Engineering Research Council of Canada (NSERC). cracking was insignificant. After cracking, the behavior of the strengthened beams significantly improved. The NSM REFERENCES FRP reinforcing bars or strips limited the deflections and ACT Committee 440, 2002, "Design and Construction of Externally crack widths. At any load level, the deflections of the Bonded FRP Systems for Strengthening Concrete Structures (440.2R-02)," strengthened beams were significantly less than that of the American Concrete Institute, Fannington Hills, Mich., 45 pp. ACI Committee 440, 1996, "State-of-the-Art Report on Fiber Reinforced unstrengthened beam; Plastic Reinforcement for Concrcte Structures (440R-96) (Reapproved 2002)," 2. The ultimate strength of the strengthened beams with American Concrete Institute, Farmington Hills, Mich., 68 pp. NSM CFRP strips was governed by the tensile rupture Alkhrdaji, 1:; Nanni, A.; Chen, G.; and Barker, M., 1999, "Solid RC Decks Strengthened with FRP," Concrete International, V. 21, No. 10, Oct., pp. 37-41. strength of the CFRP strips. A full composite action between Arduini, M.; Gottardo, R.; and De Riva, E, 2001, "FRP Rods for the NSM CFRP strips and concrete was achieved; Flexural Reinforcement of Existing Beams: Experimental Research and 3. FRP-epoxy-split failure was the dominant mode of Applications," Proceedings of the International Conference on FRP failure for the beam strengthened with NSM CFRP reinforcing ComposiJes in Civil Engineering (CICE), V. 2, Hong Kong, China, Dec. 12-15, bars as a result of high tensile stresses at the CFRP reinforcing pp. 1051-1058. Asplund, S. Q., 1949, "Strengthening Bridge Slabs with Groutedbar-epoxy interface; Reinforcement," ACT JOURNAL, Proceedings V. 52, No.6, Jan., pp. 397-406. 4. Concrete split failure was the goveming mode of failure for AS1M C 39-01, 2001, "Standard Thst Method for Compressive Strength of the beam strengthened with NSM GFRP thermoplastic strips; Cylindrical Concrete Specimens," ASTM International, West Consohocken, Fa., 5 pp. 5. Failure of beams strengthened with externally bonded Blaschko, M., and Zilch, K., 1999, "Rehabilitation of Concrete SlIUctures CFRP or thermoplastic GFRP strips was due to debonding with CFRP Strips Glued Into Slits," Proceedings of the 12th Internationalbetween the strips and the concrete. The debonding failure of Conference on Composite Materials, Paris, July 5-9. (CD-ROM)the externally bonded FRP strips was brittle and occurred at Carolin, A.; Hordin, H.; and T1Ujsten, B., 2001, "Concrete Beams Strengthened with Near Surface Mounted Reinforcement of CFRP,"a load level significantly lower than the ultimate load Proceedings of the International Conference on FRP Composites in Civilmeasured for beams strengthened with NSM CFRP reinforcing Engineering (CICE), V. 2, Hong Kong, China, Dec. 12-15, pp. 1059-1066.bars or strips and NSM GFRP thermoplastic strips; ChemCo Systems, Inc., 1995, "Kemko® 040 Dowel RegSety Grout," 6. Strengthening a concrete beam with NSM CFRP Technical Data Sheet, Nov., htlp:llwww.chemcosystems.com.reinforcing bars provided a considerably less increase of the De Lorenzis. L.; Nanni, A.; and La Tegola, A., 2000, "Flexural and Shear Strengthening of Reinforced Concrete Structures with Near Surfaceload-carrying capacity compared with similar beams Mounted FRP Rods," Proceedings of the 3rd International Conference onstrengthened with NSM CFRP strips with the same axial Advanced Composite Materials in Bridges and Stru.ctures (ACMES Ill),stiffness due to possible early debonding failure that Ottawa, Ontario, Canada, Aug. 15- 18, pp. 521-528.occurred at the CFRP reinforcing bar-epoxy interface and to De Lorenzis, L., and Nanni, A., 2002, "Bond Between Near-Surface Mounted FRP Rods and Concrete in SlIUcturaJ Strengthening," ACI Strncturalthe smaller bonding surface of the NSM CFRP reinforcing Journal, V. 99, No.2, Mar.-Apr., pp. 123·1 32.bars with respect to the NSM CFRP strips; Dow Plastic Chemical, 2000, "Fulcrum Thennoplastic Composite Tech- 7. No slip was observed for the different NSM FRP nology," Technical Data Sheer, Dec., hUp:llwww.dowfulcrum.com.reinforcing bars and strips strengthening techniques up to EI-Hacha, R.; Wight, R. G.; and Green, M. F., 2001, "Prestressed Fibre- Reinforced Polymer Laminates for Strengthening Structures," Progress inultimate load-carrying capacity; Structu.ral Engineering and Materials Journal, pp. 111-121. 8. The strength of the reinforced concrete beam strengthened EI-Hacha, R.; da Silva Filho, J. N.; Melo, G. S.; and Rizkalla, S. H.,with the NSM technique provided a significant increase of 2004, "Effectiveness of Near-Surface Mounted FRP Reinforcement forthe overall ductility of the member when compared with Flexural Strengthening of Reinforced Concrete Beams," Proceedings of 'he 4th International COliference on Advanced Composite Materials in Bridges andexternally bonded FRP strips; and S<rncru",s (ACMBS /V), Calgary, Alberta, Canada, July 2{}-23. (CD· ROM) 9. Using the same axial stiffness of FRP to strengthen Hassan, T. K., 2002, "Flexural Perfonnance and Bond Characteristics ofreinforced concrete beams, the beams strengthened with FRP Strengthening Techniques for Concrete Structures," PhD thesis,NSM FRP reinforcement achieved higher ultimate load than Department of Civil and Geological Engineering, University of Manitoba, Winnipeg, Manitoba, Canada, 304 pp.beams strengthened with externally bonded FRP reinforce- Hughes Brothers, 2002, "AsIan 200 CFRP Bars, and AsIan 500 CFRPment. This is due to the high utilization of the tensile strength Tape," Technical Information, Dec., http://www.hughesbros.com.of the FRP reinforcement. The NSM FRP strips have double Nordin, R; TaIjsten, B.; and Carolin, A., 2001, "Concrete Bcamsthe bond area compared with an externally bonded FRP Slrengthened with Prestressed Near Surface Mounted Reinforcement (NSMR)," Proceedings of the International Conference on FRP Composites instrips. It should be noted that the thickness could affect the Civil Engineering (CICE), V. 2, Hong Kong, China, Dec. 12-15, pp. 1067-1075.debonding phenomena. Structural Composites Inc. (SCI), 2002, '"Technical Guide for the Selection, In summary, the NSM FRP strengthening technique could Design and Insta11ation of Ihe En-Force FRP Systems: En-Force: Thcbe considered as a valid alternative to an externally bonded Future of Concrete Strengthening," Waller, Tex. TiiJjslen, B., and Carotin, A., 2001, "Concrete Beams Strengthened wilhFRP strengthening system and an attractive, efficient method Near Surface Mounted CFRP Laminates," Proceedings of the 51h Internatiollalfor enhancing the stiffness and increasing the flexural Conference 011 Fibre-Reinforced Plastics for Reinforced Concrete Struclllresstrength of deficiently reinforced concrete members. (FRPRCS·5), V. I, Cambridge, UK, July 16·18, pp. 107- 116.